Personally, I don't think it's worth the extra stress (as minimal as it may seem right now). Universities only consider your 8 best GCSE's, and if you've already got six 9's, that's really good as it is - most universities won't even look at your 6 in Spanish, unless you're doing a course that is specifically related to Spanish. The only benefit doing Persian will have is that, if you do get a 9, it will overtake any other non-9 grade, unless the course you do at university specifically requires a certain GCSE grade (eg. maths at grade 6 is a common one).
(This is also coming from someone who did three languages at AS (GCSE in native language, option E in Mandarin Chinese, and I'm also doing A-Level French).)
Hope that helps!